OSPF: Difference between revisions
→Troubleshooting
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There is only one way to use OSPF without an Area 0, and that is to use OSPF with a single area.
If you only have one OSPF area, you can number it any way you like, but if you have even two areas, you must have an Area 0.
* Why do we use Backbone Area?
The reason for this star-like topology is that OSPF inter-area routing uses the distance-vector approach and a strict area hierarchy permits avoidance of the "counting to infinity" problem.
OSPF prevents inter-area routing loops by implementing a split-horizon mechanism, allowing ABRs to inject into the backbone only Summary-LSAs derived from the intra-area routes,
and limiting ABRs' SPF calculation to consider only Summary-LSAs in the backbone area's link-state database.
Simple example of OSPF's distance-vector behavior:
<-- Area 5 --><-- Area 0 --><-- Area 4 -->
R5-----------R1-----------R2------------R3---------------------R4
Cost 3 Cost 5 Cost 7 Cost 12
LSA--> LSA-->
Type3 LSA Type3 LSA
{From R1} {From R2}
R5 cost is 3 R5 cost is 8
Consider what happens to a /32 Loopback route for R5:
R5 sends a Type1 LSA containing the /32 Loopback
R1 (Area 5 ABR), is connected to Area 0; it translates the Type1 LSA into a Type3 LSA with a cost of 3.
R2 (Area 4 ABR) receives R1's Type3 LSA (metric 3) and changes the metric to R5's Loopback, based on R2's cost to R1.
Now R2's Type3 LSA for R5 has a cost of 8. This is the distance-vector behavior.
Requiring all non-backbone routes to go through the backbone is a loop-prevention mechanism.
Connecting non-backbone OSPF areas at an ABR:
If 2 areas aren't connected through area 0 (discontiguous), how does OSPF behaving as a link state protocol increase the possibility of routing loops?
As we saw above, OSPF uses distance-vector behavior to send routes through the Area 0 backbone.
Distance-vector protocols have well-known limits, such as the count-to-infinity problem.
OSPF would be vulnerable to the same issues, if we didn't have boundaries on its behavior.
== Basic OSPF Lab ==
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